Revision of Houstonia (Rubiaceae-Hedyotideae)

Terrell, Edward

ABSTRACT: The genus Houstonia (Rubiaceae-Hedyotideae) comprises 20 annual and perennial herbaceous species native to the U.S.A., Canada, and Mexico. This revision utilizes seed morphology as shown by scanning electron microscopy, chromosome number, and pollen structure as the most taxo nomically significant characters. Other important morphological data have come from inflorescence types and flower and fruit morphology. The genus is subdivided into two subgenera, Houstonia and Chamisme. Subgenus Houstonia comprises two sections: sect. Houstonia includes the type of the genus, H. caerulea, and four annual and perennial species of the eastern and central U.S.A. and eastern Canada; sect. Mullera includes H. rosea, an annual species of the southcentral and southeastern U.S.A. Subgenus Chamisme also comprises two sections: sect. Amphiotis includes the type, H. purpurea, and three other perennial species of the eastern and central U.S.A. and Canada; section Ericotis includes the type, H. rubra, and nine other annual and perennial species of the southwestern U.S.A. and Mexico. The systematic treatment provides synonymies, descriptions, keys, phenological and geographic data, illustrations, and distribution maps.